The U.S. space agency NASA’s Messenger probe is scheduled to reach Mercury later this week, and become the first spacecraft to orbit the planet closest to the sun.

During Messenger’s year-long orbital mission, scientists expect to learn much more about Mercury’s unique physical characteristics and environment, including its geologic history and magnetic field, the composition of its core and, perhaps most importantly, whether there is frozen water at its poles. NASA astronomers say learning more about Mercury will help them better understand the early solar system and how the Earth was formed some 4.5 billion years ago.

Messenger was launched in August 2004, and it has taken photographs of Mercury three times during its nearly seven-year-long voyage in and around the inner solar system. It made flybys in January and October 2008, and in September 2009. Messenger’s pictures will be the first new data on Mercury since NASA’s Mariner 10 probe made three flybys in the mid-1970s. Mariner is the only other spacecraft to have visited the innermost planet.

The new images will allow scientists to map 98 percent of Mercury's surface, which is marked by Moon-like craters, smooth plains, and long, winding cliffs.

NASA says Messenger will briefly fire its main thruster and drop into orbit around Mercury at 0054 hours Universal time on Friday, March 18. The science leg of the mission will begin on Monday, April 4.

Messenger's name is an acronym for the craft's multifaceted mission: "ME" for Mercury, "S" for surface, "S" for space, "EN" for environment, "GE" for geochemistry and "R" for ranging.

Slightly larger than our moon, Mercury is the smallest planet in the solar system, but also one of the most dense, with a large iron core that comprises at least 60 percent of its total mass. That is twice the density of the Earth’s core. Mercury's average distance from the Sun is just 58 million kilometers, about two-thirds closer to the sun than Earth is. It is the only other planet in the inner solar system besides Earth with a north-and-south-pole-type magnetic field.

A single Earth year is the equivalent of four years on Mercury, which makes one lap around the sun every 88 Earth days. That makes the tiny world the "fastest" planet in the solar system. Its relatively swift movement across the evening skies might explain why early astronomers named the bright planet Mercury, after the winged-footed messenger of the gods in ancient Roman mythology. But Mercury rotates on its axis so slowly that one of its days equals 176 days here on Earth.

Mercury also has the widest temperature variations in the solar system. Because of an extremely elongated solar orbit, the surface on Mercury can range from a scorching 425 degrees Celsius to a frigid minus 185 degrees Celsius, a range of more than 600 degrees Celsius.